1. Field of the Invention
The present invention relates to a pressure-sensitive adhesive composition which can be used for the production of a pressure-sensitive adhesive tape, sheet, label, etc., and in which no or substantially no organic solvent is used. More particularly, the present invention is concerned with a pressure-sensitive adhesive composition comprising a liquid diene polymer containing one functional group at each end (terminal) thereof, an isocyanate compound containing 2 or more isocyanate groups capable of reacting with the above functional groups, thereby increasing the molecular weight of the composition, i.e., a polymerization reaction, and a tackifier containing therein at least one polar group selected from the class consisting of --COOH, --OH, and --CH.sub.2 OH, which forms a uniform adhesive layer.
2. Description of the Prior Art
Hitherto, a method in which a mixture prepared by adding appropriate agents such as a tackifier, a softening agent, a plasticizer, a pigment, etc., to a rubber based or acrylic based polymer, is dissolved in an organic solvent, the resulting pressure-sensitive solution containing about 30 weight% solids is coated on a support such as a plastic sheet, metal foil, etc., at a definite thickness, and then the organic solvent is evaporated in a heating-drying step, thereby re-forming an adhesive of the above mixture on the support in layer form, has been most widely employed to produce pressure-sensitive adhesive tapes, etc.
In accordance with such a method, however, two or more times as much organic solvent as solids is required for dissolving the solids of the adhesive and forming an adhesive layer. The organic solvent used, however, does not play any role in increasing the pressure-sensitive adhesive properties of the adhesive tape. This method, therefore, suffers from the defect that the use of a large amount of organic solvent requires a heat source, apparatus for evaporating the organic solvent, and apparatus for recovering the organic solvent, thereby increasing the cost of the product.
The inventors studied compositions capable of forming a pressure-sensitive adhesive layer on a support without using a solvent, such as a liquid composition comprising: (A) a liquid diene polymer containing functional groups at both ends thereof which is fluid at room temperature; (B) an isocyanate compound capable of reacting with the liquid diene polymer to produce a rubber-like polymer; and (C) a tackifier. As a result, it has now been found that in converting this composition on the support into an adhesive layer having uniform thickness and uniform properties by polymerization, the composition must be capable of forming a completely miscible, uniform layer, and that the kind of tackifier and the amount of the tackifier play great roles in providing a completely miscible, uniform layer.
The term "uniform layer" as used herein has an important meaning in the present invention. In prior art methods of producing a pressure-sensitive adhesive using an organic solvent, solvents having a high solvating action were naturally used so as to satisfy such uniformity requirements, and the obtaining of a "uniform layer" was a problem overcome in reaching the present invention using no organic solvent.
The reasons why the composition of the present invention should be capable of forming a completely miscible "uniform layer", i.e., a layer where there are substantially no heterogeneous portions or spots in the layer when the composition is coated on a support, will be hereinafter explained.
Firstly, when a liquid composition comprising the above three components is coated on a support at a definite thickness and heated to accelerate reaction, the viscosity of the coating layer decreases extremely (for example, 20 cps at 120.degree. C.) and is liable to flow. This is because the liquid diene polymer, a major component of the composition, has a relatively low molecular weight as described hereinafter. As a result, if there are present heterogeneous portions or spots, the heterogeneous portions or spots nucleate, thereby causing the phenomenon of voids (free of tackifier) caused by the heterogeneous portions or spots repelling the composition, or the reaction is retarded, thereby causing flow near the nuclei, and thus when the reaction is completed, areas where no adhesive layer is present and areas where the adhesive layer is especially thick are produced. Therefore, only pressure-sensitive tapes which do not have a uniform adhesive layer thickness and which are poor in appearance are obtained. Furthermore, the adhesive strength, the most important property thereof, is extremely low as the contact area between the adhesive layer and the surface to which the tape is to be stuck is low.
Secondly, where the coating layer is non-uniform, even if the components are completely miscible, the polymerization of the liquid diene polymer and the isocyanate compound does not proceed uniformly, i.e., at certain areas of the adhesive layer the reaction proceeds excessively whereas at other areas the reaction proceeds insufficiently, thereby causing stringing, which results in adhesive remaining on the surface to which the tape was stuck if the tape is peeled off. A satisfactory pressure-sensitive adhesive tape cannot be obtained in such a manner.